Abstract: A mold-core rod combination for producing a plastic preform for forming blow molded plastic bottles which comprises: a neck portion defining an opening; a tubular sidewall portion depending therefrom; and an integral base structure depending from the tubular sidewall portion to a closed end; the preform having an outside wall face and an inside wall face with one of these in the base structure having integrally formed thereon a plurality of filets, extending longitudinally of the preform and defining a continuous reinforcing ring of varying thickness spaced from the closed end and circumscribing the base structure, wherein the filets decrease progressively in width and radial thickness at least from the reinforcing ring toward the closed end. The preform is capable of forming a blow molded plastic bottle with a bottom portion having a continuous reinforcing ring of circumferentially continuous radially extending alterations in wall thickness with a regularly undulating cross-section along the circumference.